(1) Field of the Invention
The present invention generally relates to glass frit compositions. More particularly, this invention relates to lead-containing glass frit materials of the type suitable for use in wafer bonding processes, wherein the moisture resistance of the glass frit material after firing is increased by a lead phosphate coating formed on an exposed outer surface of the glass frit material.
(2) Description of the Related Art
Within the semiconductor industry, there are numerous applications that require bonding a semiconductor wafer to a second wafer or glass substrate. As an example, a microelectromechanical system (MEMS) device formed in or on a semiconductor wafer (referred to herein as a device wafer) is often capped by a semiconductor or glass wafer (referred to herein as a capping wafer), forming a package that defines a cavity within which the MEMS device, such as a suspended diaphragm or mass, is enclosed and protected. Examples of MEMS devices protected in this manner include accelerometers, rate sensors, actuators, pressure sensors, etc. By the very nature of their operation, MEMS devices must be free to move to some degree, necessitating that the seal between the wafers is sufficient to exclude foreign matter from the cavity. Certain MEMS devices, such as absolute pressure sensors, further require that the cavity be evacuated and hermetically sealed. The performance of motion sensors and accelerometers with resonating micromachined components also generally benefit if the cavity is evacuated so that their micromachined components operate in a vacuum. A hermetical seal also ensures that moisture is excluded from the cavity, which might otherwise form ice crystals at low temperatures that could impede motion of the MEMS device.
In view of the above, the integrity of the bond that secures the capping wafer to the device wafer is essential to the performance and life of the enclosed MEMS device. Various bonding techniques have been used for the purpose of maximizing the strength and reliability of the wafer bond. Such techniques include the use of various intermediate bonding materials, including glass frit, as well as silicon direct and anodic bonding techniques that do not require an intermediate bonding material. As would be expected, each of these bonding techniques can be incompatible or less than ideal for certain applications. Silicon direct and anodic bonding methods require very smooth bonding surfaces, and therefore cannot produce a vacuum seal when trench isolation or unplanarized metal crossunders are employed on the device wafer, such as to electrically interconnect a MEMS device to bond pads outside the vacuum-sealed cavity of the package. In contrast, glass frit and other intermediate bonding materials are able to form suitable bonds with deposited layers, runners and other surface discontinuities often found on device wafers.
Glass frit bonding materials used for wafer bonding are often deposited by a screen printing technique, in which case the material is deposited as a paste that contains a particulate glass frit material, a thixotropic binder, and a solvent for the binder. The proportions of glass frit, binder and solvent are adjusted to allow screen printing of a controlled volume of the paste on a designated bonding surface of one of the wafers, typically on the capping wafer. After firing, the capping and device wafers are aligned and then mated so that the glass frit particles (bonded together as a result of the firing operation) contact a complementary bonding surface of the second (e.g., device) wafer. The wafers are then incrementally heated to completely remove the solvent and binder and finally melt the glass frit, so that on cooling the glass frit material resolidifies to form a substantially homogeneous glass bond line between the wafers.
The composition and size of a glass frit material used in a wafer bonding process are typically chosen on the basis of process and compositional considerations, including screening properties, process temperatures, etc. Suitable glass frit materials are usually a mixture of various oxides, such as litharge (PbO; also known as lead oxide, yellow and lead monoxide), boric acid (H3BO3) which serves as a source for boron oxide (B2O3), silicon dioxide (SiO2; silica), aluminum oxide (Al2O3, alumina), titanium oxide (TiO2, titania), cupric oxide (CuO), manganese dioxide (MnO2) or manganese carbonate (MnCO3) as a source for manganous oxide (MnO), calcia (CaO), lithium oxide (Li2O), ceria (CeO2), cobaltous carbonate (CoCO3), and others. Glass frits that contain lead are widely used in the semiconductor industry to enable silicon wafer bonding at temperatures sufficiently low to reduce the risk of thermal damage. However, the percentage of lead required in a glass frit material to achieve high quality wafer-level sealing at low firing temperatures makes the resulting glass susceptible to attack by moisture. When a lead-containing glass is exposed to moisture, lead can be released from the glass and, in concert with moisture, migrate to exposed aluminum metallization on the wafer, resulting in galvanic corrosion of the metallization. This corrosion is a source of concern for both the wafer-level yield and the system-level reliability of a semiconductor device.
Prolonged exposure to moisture can also create holes in a glass bond line that are potential paths for the ingress of air, moisture and contaminants into a cavity sealed by the bond line. Glass frit used in wafer bonding processes is often exposed to attack by moisture as a result of the use of water to cool the wafers and remove debris during the dicing operation used to singulate dies from the wafers. During singulation, the glass bond line is generally covered by water for the duration of the dicing operation, making the glass-water interaction difficult to avoid. Following dicing, if the device is placed in a package that is not hermetically sealed, the glass bond line may be subjected to prolonged exposure to moisture during the operating life of the device.
In view of the above, it would be desirable if lead-containing glass frit materials of the type used in wafer bonding processes could be rendered more resistance to attack by moisture.